In order to obtain correct wiping of the swept surface, it is necessary that the wiping strip of the wiper blade should be applied on the swept surface with a substantial pressing, or wiping, force. To this end at least one spring is usually provided, for example a helical tension spring. This spring is disposed between the driving head and the wiper arm, and in practice it applies a substantially constant nominal wiping torque to the wiper arm about the pivot axis of the latter, so as to urge the wiping strip of the blade into contact with the swept surface.
The design of this wiping apparatus is such that the wiping strip is applied not only during the wiping movement over the swept surface, but also when the windshield wiper is in its rest or parked position. Because this nominal wiping force is exerted on the wiping strip at all times, even when the wiper is parked, it is found that the strip adopts and retains a residual deformation resulting from the bending of its profile against the swept surface. This results in a deterioration in the effectiveness of the wiping operation. It is accordingly desirable to be able to reduce the nominal wiping force which is applied to the wiping strip when the wiper is parked.
In addition, the quality of the wiping operation depends in operation on the magnitude of the wiping force applied to the wiping strip, in particular as a function of the speed of travel of the vehicle, and as a function of the frequency of the oscillating movement of the wiper itself.
It is known to augment the wiping force by the use of aerodynamic attachments which are mounted on the wiper arm. However, these devices do not enable the value of the wiping force to be controlled with any precision. In particular, they do not provide any facility for making controlled adjustments to the value of this force in response to any operating parameter of the vehicle.
In order to overcome the drawbacks mentioned above, it has already been proposed to provide a screen wiping apparatus of the type mentioned above, in which the means which are arranged between the driving head and the wiper arm, so as to apply a wiping torque to the latter, include at least one spring which applies to the wiper arm a nominal wiping torque that is substantially constant, together with a means for applying a complementary torque to the wiper arm. The algebraic value of this complementary torque is adjustable in such a way that the value of the wiping torque can be either increased or reduced, according to (in particular) an operating parameter of the vehicle.
The complementary torque applying means includes, for example, an actuator which is associated with the driving head, and which includes an output member (typically in the form of a ram) which is connected to the wiper arm. Although such an arrangement is to a great extent satisfactory, it is found that the whole of the apparatus is subjected to cyclic forces, which result in particular from the curved shape of the windshield. While the wiper is operating, this curved shape gives rise to cyclic pivoting, at a small amplitude, of the wiper arm about its pivot axis. These oscillations of the wiper arm result in cyclic stresses being applied to the actuator, and especially to the anchorage of the body of the actuator on the driving head.